Some water-soluble polymers such as cellulose ethers are difficult to dissolve in water due to the fact that the first particles that come into contact with water immediately swell and stick to each other, forming a gel-like barrier that shields the remaining polymers from hydration. These water-soluble polymers are conveniently supplied as a particulate dry material that is then dissolved in water for the desired end use of such water-soluble polymers. The above-described gel-blocking behavior of water-soluble polymers is a considerable drawback for those applications of water-soluble polymers that comprise the solution of the particulate water-soluble polymer such as cellulose ethers in aqueous systems.
One approach used in industry to overcome this problem, if permissible in the end-use application, is to apply glyoxal to the cellulose ether to form a hydrolytically-unstable network. The crosslinking of the cellulose ether with glyoxal is therefore reversible in aqueous medium and thus treated cellulose ether can be suspended in aqueous medium and ultimately dissolved when the crosslinked network formed with glyoxal is hydrolytically cleaved. The drawback of this method is that glyoxal is considered as a toxic compound and is regenerated upon hydrolysis of the crosslinked network. Thus, alternatives avoiding the above-described gel-blocking behavior are desired.
U.S. Pat. No. 3,362,847 discloses a process for improving the water-dispersibility of water-soluble cellulose ether by treating the surface of the particulate cellulose ether with a combination of a water-soluble polybasic organic carboxylic acid having from 2 to 10 carbon atoms and a water-soluble organic polyamine having at least two primary amino groups. Preferably, the polybasic acid and amine are applied to the cellulose ether by dissolving the polybasic organic carboxylic acid and the water-soluble organic polyamine in a solvent, which is a non-solvent for the cellulose ether, and suspending the cellulose ether in such treating solution.
U.S. Pat. No. 3,461,115 relates to a process for the preparation of a macromolecular compound containing hydroxyl groups, which is soluble in water without forming lumps. This process comprises treating the water-soluble macromolecular compound in the solid state with 0.5 to 5% by weight of an aliphatic dicarboxylic acid containing 2 to 8 carbon atoms, or a salt or an ester thereof.
GB 1,017,746 describes a method of producing a crosslinked product from cellulose or a cellulose derivative, which comprises reacting a solution or suspension of the cellulose or cellulose derivative in an organic liquid with the anhydride of a tetra- or higher basic carboxylic acid in the presence of an organic is base containing nitrogen. This method allows preparing clear and transparent products in a simple way. According to the examples the cellulose derivative is dissolved in an organic solvent such as acetone and the crosslinking reaction results in a stiff, more or less transparent gel. This reference neither discloses the preparation of a water-soluble particulate polymeric material nor addresses the problem of gel-blocking when dissolving such a polymeric material.
US 2005/0143572 relates to a method for the production for cellulose ethers whereby the cellulose ethers having free hydroxyl groups are reacted with dicarboxylic and/or polycarboxylic acids and a nitrogen-containing compound. The process comprises intensively mixing essentially dry, pulverulent cellulose ether with a mixture of organic bifunctional and/or polyfunctional acid and nitrogen-containing compound in a non-nucleophilic organic solvent prior to reacting the cellulose ether to provide the modified cellulose ether, which can be stirred into water at a pH greater than or equal to 11 without agglutination.
The object of the present invention is to provide a process for the preparation of a particulate reversibly crosslinked polymeric material that can be effectively performed under mild conditions that result in delay of the dissolution of the water-soluble polymer in an aqueous system even at lower crosslinker levels. Another goal of the present invention is to avoid formation of by-products upon dissolution of the water-soluble polymers that may cause a health concern so that the products of the present invention can also be used in food, personal care or pharmaceutical applications.